Phenolic-epoxy adhesives have been known for over 50 years and were one of the first high temperature adhesives to become commercialized. Once cured, the material retains its adhesive properties over a large range of temperatures, has high shear strengths, and is resistant to weathering, oil, solvents, and moisture. The adhesive is available commercially as either a 1-part adhesive or 2-part adhesive and is available in several forms, such as pastes, solvent solutions, and supported films. Of the three forms, the adhesive film generally provides better adhesive strength
In a 1-part adhesive, both phenolic and epoxy are combined by the manufacturer and is available to the consumer as a single component. Although convenient, as no mixing is required by the consumer, the adhesive suffers from a shortened useable lifetime, also known as shelf-life, at room temperature because the highly reactive phenolic immediately initiates the cure chemistry of the epoxy once it is added. This effect is even more pronounced at temperatures above room temperature, where the useable lifetime of the adhesive can be reduced to minutes. In the special case of supported films, the shelf life is so short that films must be stored under refrigerated conditions before use, as described by Petrie in “Epoxy Adhesive Formulations”, McGraw-Hill publishers, 2006. In addition, the manufacturer of a 1-part adhesive and can also experience a reduced time to manufacture the adhesive because the phenolic quickly begins to cause an increase in solution viscosity as it starts to react with the epoxy, and, if care is not taken, the viscosity will continue to increase until a point is reached at which the processability is severely compromised.
In a 2-part adhesive, the phenolic and epoxy are separated and supplied by the manufacturer as two distinct packages, which are then combined and mixed by the consumer immediately before use. Because the phenolic and epoxy are kept separate, the adhesive does not suffer from a reduced shelf-life and it is therefore not necessary to store under refrigerated conditions; however, it does have a reduced green time, the time during which after the phenolic and epoxy are mixed the adhesive remains workable. As in the case for the reduced manufacturing time for a 1-part adhesive, the viscosity of a 2-part adhesive continues to increase after the phenolic is combined with the epoxy until a point is reached at which the adhesive gels and begins to set. The consumer therefore, has a finite period of time during which the adhesive remains workable.
Phenolics are also reactive toward benzoxazines and have been used to decrease the polymerization temperature, as demonstrated in U.S. Pat. No. 6,207,786, of Ishida et al., where the polymerization temperature was lowered from 190° C. to 145° C. after the addition of phenol. The phenolic in this case functions as a polymerization initiator, or curative, for benzoxazine, in a similar manner as phenols initiate the cross-link chemistry of epoxies.
U.S. Pat. No. 6,207,786, to Ishida et al. and U.S. Pat. No. 6,437,026 to Garrett have described the use of phenolics to reduce both the cure temperature and cure time when it is used in concert with benzoxazines and epoxies. The resulting thermosetting adhesives have a Tg in excess of 180° C., with lower water uptake and good adhesive properties, desirable properties for applications involving electronics. These inventions have shortened useable lifetimes, as the compositions described contain a combination of phenolics and epoxies, which will start to cure immediately after being added together.
There remains a need to improve the useable lifetime of phenolic-epoxy, phenolic-benzoxazine, and phenolic-epoxy-benzoxazine mixtures whereby the phenolic is a protected phenolic and in the form of an aryl glycidyl carbonate. The phenolic group may be released in controllable ways by for examples heat, radiation, base or acid catalyzed reactions, nucleophilic substitution reactions and combination thereof to facilitate the crosslinking or polymerization of the epoxides and/or benzoxazines. Although other protected phenolics are known, such as aryl alkyl carbamates as described by U.S. Pat. No. 4,123,450 of Weber and WO 87/05600 of Chan, they have distinct disadvantage that for every equivalent of phenolic generated one equivalent of a highly toxic alkyl isocyanate is also created. In many instances, the alkyl isocyante is a gaseous compound that is harmful and detrimental to human health, in addition to performing as a low molecular weight contaminate that is free to migrate and cause a decrease in the performance of the adhesive. In addition, said contaminate would perform as a plasticizer thereby causing an unwanted decrease in Tg. In contrast, the only byproducts resulting from the deprotection of aryl glycidyl carbonates are non-toxic CO2 and a reactive epoxy. Unlike the low molecular weight contaminate just described, the advantage of a reactive epoxy as a byproduct is that is becomes incorporated into the adhesive matrix during the curing process and it would therefore not cause a decrease in Tg nor adversely affect the performance of the adhesive The mixtures described above would have benefit in composites, molding compounds, adhesives and coatings, particularly for electronic applications, which include, but are not limited to underfill materials, electronic packaging, encapsulation, die attach adhesives, conductive adhesives, lead free solders, anisotropic conducting films (ACFs) and non-adhesive conductive films (NCFs). Said compositions would find benefit in electronic displays, circuit boards, flip chip, and other semiconductor devices.